Tweet This

File this under "counterintuitive:" Under certain conditions, scientists and curious folks alike have observed hot water freezing faster than cold water. This effect is more than just surprising. It seems to fly in the face of what’s known about chemistry and physics. Not everyone is convinced that this effect — known as the Mpemba effect — is real. This is the kind of mystery that makes researchers roll up their sleeves.

Shutterstock

Here's what we know: The Mpemba effect gets its name from Erasto Mpemba, a student in Tanzania. In 1963, he noticed the effect while making ice cream — his container of boiling-hot milk and sugar froze before a classmate’s cooler mixture. His teacher was skeptical, but Mpemba eventually published a paper about the phenomenon with a visiting professor, which led to the effect being named in his honor. Score one for youthful curiosity.

Mpemba was in good company. Aristotle, Descartes, and other leading minds of history also noticed this weird effect and wrote about it. Since Mpemba published his work, many fascinated researchers have followed up on it.

But there's plenty we don't yet know. The phenomenon of "hot water freezes faster than cold" is not something that's easy to test scientifically. What should scientists be looking for, exactly? Should they aim to see which container reaches 0 degrees Celsius (the freezing point of water) first? Or to see which container freezes all the way solid first? Or something else? Some researchers have tried to reproduce the Mpemba effect in a consistent way and did not observe the effect at all — most recently in an account published last November in the journal Scientific Reports. Ideally, someone would be able to set up conditions that consistently give the same effect over and over. That would be a very good indication that there is a scientific basis for that effect. Unfortunately, there's no consensus on when the Mpemba effect happens.

The thing is, if someone could find reproducible conditions for the Mpemba effect, it could teach us something we don't yet know about the chemistry and physics of water, which is already one of the weirdest, most wonderful molecules we have. Many scientists have attempted to offer explanations for what Mpemba saw, although none of the explanations seems to satisfy everybody.

One argument is that hot water evaporates, decreasing the volume left to freeze. Another is that hot and cold water can contain different amounts of dissolved impurities and gases, factors that can affect freezing. Yet another explanation implicates convection currents, the movement of water molecules due to non-uniform temperature distribution in a container. In 2012, the Royal Society of Chemistry held a competition to find the best explanation for the Mpemba effect. The winner -- who was skeptical at first but eventually saw the effect in his own experiments -- believed that all of the above factors are involved, but also theorized that when someone’s seeing the Mpemba effect, it’s because the cooler water has become supercooled, a weird physical state that’s colder than the freezing point yet not frozen solid.